Such a keyboard encoding device is taught by. U.S. Pat. No. 4,725,816. When a given key is actuated resistances are mutually coupled so as to generate, together with a current source, a d.c. voltage that is unique to the actuated key. An A/D converter converts the unique d.c. voltage to a digital signal. One problem with this solution is that the A/D converter is a relatively expensive and complicated component. Another problem is that the A/D converter is difficult/expensive to integrate in the same silicon chip as the remaining digital circuits.
A device that is very similar to the device described in U.S. Pat. No. 4,725,816 is designed to mutually couple resistances when a given key is actuated, such that said coupled resistances will form with a voltage source a voltage divider that delivers a unique d.c. voltage corresponding to the actuated key. An A/D converter converts the unique d.c. voltage to a digital signal. The above-mentioned problem encountered with the solution taught by U.S. Pat. No. 4,725,816 also applies to this solution.
U.S. Pat. No. 4,327,255 teaches a signal generating device for a telephone keypad. Actuation of a key results in the generation of a signal that consists of two sinusoidal signals of different frequencies (Dual Tone Multi Frequency, DTMF). This known device, however, functions to generate a multi-frequency signal that corresponds to a given key and that is sent, for instance, to a telephone exchange for the purpose of identifying one of a number of numbers in a subscriber identity.
U.S. Pat. No. 4,315,108 teaches a DTMF signal generating device which is implemented in one single integrated circuit.
U.S. Pat. No. 3,949,639 teaches an electronic musical instrument provided with a keyboard. Actuation of a key generates a signal of given voltage which controls an oscillator to produce a tone of given frequency.
U.S. Pat. No. 4,412,209 describes a device which in response to activation of one of a number of switches produces a unique combination of resistances and capacitances that can be detected by an external circuit connected to the device and therewith determine which key has been actuated.